Paper that has not been treated to increase its strength disintegrates readily on immersion in water, largely because the hydrogen bonds acting between paper fibers in the dry state are destroyed. It is well known that treatment by certain polymers can confer wet strength properties to paper. The wet strengthening effect can be only temporary, i.e., the disintegration of paper may be delayed on immersion in water, or the strengthening effect can be permanent. If the wet strengthening effect is permanent, the paper retains perhaps 15% or more of its tensile strength on lengthy immersion in water compared to its tensile strength in the dry or near dry state.
The wet strengthening of paper is to be distinguished from such known processes as the sizing or waterproofing of paper that reduce the rate of, or eliminate, respectively, the absorption of water by paper fibers. Many of the industrial and domestic uses of wet strengthened paper require high levels of paper wet strength together with rapid rates of water absorption, e.g., in paper tissue and toweling.
Known polymers capable of producing permanent wet strength in paper include formaldehyde-based resins such as urea- and melamine-formaldehyde resins, and the 3-hydroxyazetidinium ion-containing resins. The latter, collectively referred to in this specification as Type I polymers, are particularly useful for making permanently wet strengthened paper for several reasons: they confer relatively high levels of wet strength based on a given dry mass ratio of resin to paper fiber, they do not impart an undesirable rough feel to the resulting paper, and they confer paper wet strength that is relatively permanent on treatment with aqueous solutions of low pH.
These Type I polymers can be classified into two main classes, A and B, depending on the nature of the precursors from which they are made. Class A polymer precursors are polyalkylene polyamines. Class B polymer precursors are a further class of polyamines, the polyamidoamines. Both Class A and Class B polymer precursors are reacted with an epihalohydrin to produce the Type I polymers.
While Type I polymers are effective by themselves for imparting wet strength to paper, they possess certain shortcomings in relation to performance and utility. In particular, the wet strength as measured by the paper wet tensile strength is inadequate for some purposes. Previous attempts to address this and other shortcomings have met with some success. In particular, the joint use of Type I polymers with sodium carboxymethylcellulose, as described in U.S. Pat. No. 5,316,623, is known to promote the wet and dry strength of paper compared to the use of a Type I wet strength resin alone.
In practice, however, the use of sodium carboxymethylcellulose has met with resistance from commercial papermakers because of its high viscosity in aqueous solution, e.g., aqueous solutions of 2% by weight have viscosities in excess of 1000 mPa.s (milliPascal.seconds) Dilute solutions having a viscosity of 1000 mPa.s and above are expensive to transport, based on the active ingredient content of the solution, and difficult to pump. An obvious alternative, the transport of solid grades or dispersed grades, requires the end user to dissolve and make solutions of the required concentration of sodium carboxymethylcellulose, which is time, energy and labor intensive. Moreover, the increased viscosity imparted by such a polymer results in a slower rate of drainage of the fiber slurry during papermaking and a slower rate of paper manufacture.
Another factor that adversely affects the use of sodium carboxymethylcellulose to promote the performance of Type I resins, is the quite different chemical plant and chemical starting materials required to manufacture the two types of polymers.
A further factor that adversely affects the use of sodium carboxymethylcellulose to promote the performance of Type I wet strength resins is that quite dilute aqueous blends of the two polymers have been found to increase in viscosity, probably through polymer network formation and/or polyelectrolyte complex formation. This practically precludes the formulation of a single solution blend of a Type I polymer and sodium carboxymethylcellulose.
There is therefore a need for a polymer that can provide the following advantages:
Enhancing the wet strength properties of Type I polymers by providing a polymer that possesses a viscosity of less than about 1000 mPa.s at a polymer solids concentration of at least about 10% at a temperature of 25.degree. C. PA1 Enhancing wet strength properties with the potential capability of sharing common starting materials, intermediates, manufacturing plant, and process steps. PA1 The capability, in a polymer that has carboxyl groups, of forming more stable aqueous solution blends with a Type I polymer than does sodium carboxymethylcellulose.